1. Field of the Invention
The present invention relates to an article comprising a holographic medium between a pair of substrates and having an environmental barrier seal, as well as a process for preparing same.
2. Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index imprinted into a storage medium. Holographic systems are discussed generally in Psaltis et al., “Holographic Memories,” Scientific American, November 1995.
The capabilities of typical holographic recording systems are determined in part by the storage medium. One type of holographic recording media used recently for such systems are photosensitive polymer films. See, e.g., Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, (Los Angeles, Calif., 1990), pp.: 1212–03. The holographic recording media described in Smothers et al., supra contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing (recording) of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting diffusion create a refractive index change, thus forming the holographic grating (hologram) representing the data.
Photosensitive polymer films are considered attractive recording media candidates for high density holographic data storage. These films have a relatively low cost, are easily processed and can be designed to have large index contrasts with high photosensitivity. These films can also be fabricated with the dynamic range, media thickness, optical quality and dimensional stability required for high density applications. See L. Dhar et al., “Recording Media That Exhibit High Dynamic Range for Holographic Storage,” Optics Letters, 24, (1999): pp. 487 et. seq.
The polymer materials used in the holographic recording medium are typically sandwiched between two glass or plastic plates or substrates to insure high optical quality. An area of disadvantage for these photopolymer films, even when sandwiched between glass or plastic substrates, is that the exposure to the various environmental factors can negatively affect the properties of these films. For example, exposure of these photopolymer films to an oxygen-containing environment (e.g., air), as well as moisture, may cause degradation of the photopolymer film, e.g., by reaction with the materials in the film, thus potentially causing permanent damage thereto. Such degradation may come in the form of reduced dynamic range of the photopolymer film and hence reduced storage of data, as well as reducing shelf life and archival life of the film.
Environmental exposure, specifically moisture, may also result in the photopolymer absorbing moisture, thus causing the photopolymer to swell and undergo refractive index changes in a spatially non-uniform fashion. When the photopolymer film is sandwiched between glass or plastic substrates, the non-uniform swelling and/or refractive index change in the polymer may diminish the optical quality of the holographic recording medium, e.g., by changing the optical properties of the film, including optical flatness. See, for example, U.S. Pat. No. 6,160,645 (Chandross et al.), issued Dec. 12, 2000 (hermetic seal comprising metal foils attached to plates having photosensitive polymer therebetween); and U.S. Pat. No. 6,671,073 (Hegel), issued Dec. 30, 2003 (holographic optical data storage device that includes photopolymer between upper and lower substrates having first and second peripheral edges respectively with an opening formed therebetween, and a connection member engaged to the upper and lower substrates wherein the connection member seals the opening).
Accordingly, what is needed is a way to protect a holographic recording medium sandwiched between such substrates against environmental factors, including moisture and oxygen, that can adversely affect the properties of the medium, including degradation of the dynamic range of the medium, as well as the optical quality of the medium.